The effect of dehydration on performance varies across different weather conditions, with dehydration during exercise in the heat provoking larger performance decrements than similar activity in cooler conditions due to the combined effects of heat and dehydration1. Similarly, different environmental conditions alter the risk and rate of you becoming dehydrated. Furthermore, there are many people who have to work under varying environmental conditions, such as miners and military personnel, where the impact of dehydration on work output can be substantial. Understanding the risks and being conscious of your hydration practices are important components of optimising performance at work and on the field.

Fluid intake in the heat

Both dehydration AND exercising in the heat have independent effects on heart rate, body temperature regulation, concentration and performance - in combination, the effects are additive 2,3. The effects of dehydration tend to be progressive (i.e. the larger the dehydration, the greater the negative effect on performance). In extreme cases, prolonged exertion in the heat combined with dehydration can increase the risk of heat stroke and heat illness, and even acute kidney failure resulting from the breakdown of muscle contents3.

Knowing how your own body responds when exercising in the heat is very important and will provide a baseline. Commencing exercise well hydrated, and maintaining a fluid intake pattern which matches sweat losses as closely as tolerable (i.e. without causing gastrointestinal discomfort), are essential for minimising the degree of dehydration incurred during exercise in the heat. In the heat, consideration should also be made for active cooling strategies, such as ice towels, ice vests, and cool water sprays. Use the POWERADE Hydration Calculator to determine fluid required for your particular exercise or sport. For extreme conditions use your sweat rate to guide fluid intake (see "How To Determine Your Personal Sweat Rate")

Fluid intake in the cold

The risk of dehydration during exercise in cooler weather conditions can be as high as in hot conditions. Many sports are played indoors, and/or people train with more clothes on, so their actual sweat rates can be close to those in warmer conditions. In contrast, most people drink much less in cooler conditions. The net result of less fluid but similar sweat rate can lead to similar levels of dehydration being incurred in cooler climates to those in warmer conditions. For example, whilst the average sweat rate for footballers in summer training is higher than winter training (1.46 versus 1.13 L/hr), the fluid intake during training during winter was less than half that in summer (650 ml/hr in summer versus 280 ml/hr in winter), so the overall dehydration incurred was slightly higher (1.59% in summer, 1.62% in winter)3. However, evidence suggests that for the same level of dehydration, there is more impact on performance in hotter rather than cooler conditions 4.

So, the important message for those exercising in cool environments is to still pay attention to fluid intake as substantial dehydration can still occur. Individuals need to be aware of their sweat losses when exercising in the cold, and to drink according to their sweat rates.

Fluid intake at altitude

Many sports are undertaken at higher altitudes - such as winter sports, mountain climbing, and aerial sports. Athletes can also take advantage of training at higher altitudes to help boost their performance in critical events. At altitude, the air is thinner in terms of oxygen supply, but is also drier, resulting in more fluid being evaporated from the body passively (from the airways and the skin). This is the reason why people get dry throats and cracked lips in the first few days of being at altitude. There is evidence that fluid shifts around the body contribute to acute mountain sickness (altitude sickness) 5.

In addition to drier air, higher altitudes tend to be cooler, which as discussed earlier, reduces the drive to drink. Therefore, increasing the volume of fluid consumed to counteract the increased dehydration of altitude is important. The focus should be on maintaining urine output (pale in colour), as well as ensuring fluid lost during exercise is adequately compensated.

Fluid intake when travelling

Many forms of travel, such as airlines and air-conditioned buses, involve sitting in much drier air than that to which most of us are usually exposed. This "dry air" promotes greater fluid loss than being in humid air, mainly from the skin and airways via evaporation. Hence, although individuals are not exercising, fluid intake should be sustained at a consistent rate to ensure arriving well hydrated. The other benefit to sustaining hydration during travel is that it generally results in more toilet visits, which helps keep muscles and circulation moving as you walk around.